Zn-Al-Mg-BASED HOT-DIP PLATED STEEL SHEET

The present invention relates to a Zn—Al—Mg-based hot-dip plated steel sheet having an external appearance close to white.

Priority is claimed on Japanese Patent Application No. 2022-094356, filed Jun. 10, 2022, the content of which is incorporated herein by reference.

A hot-dip plated steel sheet is used as a steel sheet having good corrosion resistance. A hot-dip galvanized steel sheet, which is a representative example of the hot-dip plated steel sheet, is widely used in various manufacturing industries such as the fields of automobiles, home appliances, and building materials. In addition, a highly corrosion-resistant hot-dip galvanized steel sheet obtained by incorporating Al or Mg into a hot-dip galvanized layer has been proposed for the purpose of further improving the corrosion resistance of the hot-dip galvanized steel sheet. For example, Patent Documents 1 to 3 propose a Zn—Al—Mg-based hot-dip plated steel sheet.

The Zn—Al—Mg-based hot-dip plated steel sheet mainly contains four types of phases and microstructures of an [Al phase], a [Zn phase], a [MgZnphase], and an [Al/MgZn/Zn ternary eutectic structure] in the hot-dip plated layer. When Si is contained in the hot-dip plated layer in addition to Zn, Al, and Mg, mainly five types of phases and microstructures including a [MgSi phase] are contained in addition to the above four types of phases and microstructures. As described above, since various phases and microstructures are present in a mixed state in the hot-dip plated layer of the Zn—Al—Mg-based hot-dip plated steel sheet, the surface of the hot-dip plated layer has a satin-like external appearance.

Like a hot-dip galvanized steel sheet, a Zn—Al—Mg-based hot-dip plated steel sheet is widely used in various manufacturing industries such as the fields of automobiles, home appliances, and building materials. In recent years, there has been an increasing demand from consumers for the surface external appearance of a plated steel sheet, and a Zn—Al—Mg-based hot-dip plated steel sheet is required to have an external appearance that is not completely white but is closer to white. In the case of an external appearance close to white, there is an advantage that a defect on the surface of the plated layer is less noticeable.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Zn—Al—Mg-based hot-dip plated steel sheet in which an external appearance of a surface of a hot-dip plated layer is closer to white than in the related art, a surface defect is less noticeable, and corrosion resistance is also excellent.

In order to solve the above problem, the present invention adopts the following configurations.

[1] A Zn—Al—Mg-based hot-dip plated steel sheet, including a steel sheet and a hot-dip plated layer formed on a surface of the steel sheet, wherein

[2] A Zn—Al—Mg-based hot-dip plated steel sheet, including a steel sheet and a hot-dip plated layer formed on a surface of the steel sheet, wherein

[3] The Zn—Al—Mg-based hot-dip plated steel sheet according to [1] or [2], wherein the area fraction of the [Zn phase] of the plating microstructure in at least one of the cross sections is 10% or more.

[4] The Zn—Al—Mg-based hot-dip plated steel sheet according to [1] or [2], wherein an average grain size of the [Zn phase] of the plating microstructure in at least one of the cross sections is 2.5 to 10 μm.

[5] The Zn—Al—Mg-based hot-dip plated steel sheet according to [3], wherein an average grain size of the [Zn phase] of the plating microstructure in at least one of the cross sections is 2.5 to 10 μm.

[6] The Zn—Al—Mg-based hot-dip plated steel sheet according to [2], wherein the hot-dip plated layer has an average composition containing the group A in terms of mass %.

[7] The Zn—Al—Mg-based hot-dip plated steel sheet according to [2], wherein the hot-dip plated layer has an average composition containing the group B in terms of mass %.

According to the present invention, it is possible to provide a Zn—Al—Mg-based hot-dip plated steel sheet in which an external appearance of a surface of a hot-dip plated layer is closer to white than in the related art, a surface defect is less noticeable, and corrosion resistance is also excellent.

The present inventors investigated in detail a plated layer of a conventional Zn—Al—Mg-based hot-dip plated steel sheet having a satin-like external appearance. The satin-like external appearance appears due to coexistence of a fine glossy portion exhibiting metallic gloss and a fine white portion exhibiting white in a mixed state. Among them, when the microstructure of the hot-dip plated layer in the white portion was examined, it was found that the ratio of the [Zn phase] to the total of the [Zn phase] and the [Al/MgZn/Zn ternary eutectic structure] is higher than that in the glossy portion. In addition, it was found that the proportion of the [Zn phase] in the hot-dip plated layer is also relatively high.

Therefore, the present inventors intensively studied in order to obtain a white external appearance as a whole by increasing the white portion and decreasing the glossy portion in the hot-dip plated layer, and found that the surface external appearance of the hot-dip plated layer exhibits white as a whole by adjusting the chemical composition of the hot-dip plated layer and increasing the proportion of the [Zn phase]. In particular, the present inventors found that it is effective to increase the ratio of the [Zn phase] to the total of the [Al/MgZn/Zn ternary eutectic structure] and the [Zn phase] in the entire hot-dip plated layer.

Hereinafter, a Zn—Al—Mg-based hot-dip plated steel sheet according to an embodiment of the present invention will be described.

The Zn—Al—Mg-based hot-dip plated steel sheet of the present embodiment includes a steel sheet and a hot-dip plated layer formed on a surface of the steel sheet, and the hot-dip plated layer contains, as an average composition, Al: 5 to 22 mass % and Mg: 1.0 to 10 mass %, with a remainder including Zn and impurities, and in a case where a 5 mm square cross section parallel to a surface of the hot-dip plated layer is exposed at any position of a 3t/4 position, a t/2 position, and a t/4 position from the surface of the hot-dip plated layer with a thickness of the hot-dip plated layer represented by t, a ratio (B/A (%)) of an area fraction B of a [Zn phase] to a total area fraction A of a [Zn phase] and an [Al/MgZn/Zn ternary eutectic structure] of a plating microstructure in at least one of the cross sections is 20% or more. Here, the “5 mm square cross section parallel to a surface of the hot-dip plated layer” refers to a square-shaped exposed surface that is parallel to the surface of the hot-dip plated layer and has a 5 mm square size in plane view.

The material of the steel material as a base of the hot-dip plated layer is not particularly limited. As the material, it can be applied to general steel, Al-killed steel, and some high-alloy steels, and the shape is not particularly limited. The steel material may be subjected to Ni pre-plating. The hot-dip plated layer according to the present embodiment is formed by applying a hot-dip plating method described later to a steel material.

Next, the chemical composition of the hot-dip plated layer will be described.

The hot-dip plated layer according to the present embodiment contains, as an average composition, Al: 5 to 22 mass % and Mg: 1.0 to 10 mass %, with the remainder including Zn and impurities.

Further, the hot-dip plated layer may contain one or two selected from the group consisting of group A and group B below.

The amount of Al is in the range of 5 to 22 mass % as an average composition. Al is an element necessary for ensuring corrosion resistance. When the amount of Al in the hot-dip plated layer is less than 5 mass %, the effect of improving corrosion resistance is insufficient. When the amount of Al in the hot-dip plated layer exceeds 22 mass %, corrosion resistance may be deteriorated although the cause is unknown. From the viewpoint of corrosion resistance, the lower limit of the Al content in the hot-dip plated layer is preferably 6 mass % or more, and more preferably 11 mass % or more. The upper limit of the Al content in the hot-dip plated layer is preferably 20 mass % or less, and more preferably 19 mass % or less.

The amount of Mg is in the range of 1.0 to 10 mass % as an average composition. Mg is an element necessary for improving the corrosion resistance of the hot-dip plated layer. When the amount of Mg in the hot-dip plated layer is less than 1.0 mass %, the effect of improving the corrosion resistance is insufficient, and when the amount of Mg in the hot-dip plated layer exceeds 10 mass %, the occurrence of dross in a plating bath becomes significant, and it becomes difficult to stably manufacture a plated steel material. From the viewpoint of the balance between the corrosion resistance and the occurrence of dross, the lower limit of the Mg content in the hot-dip plated layer is preferably 1.5 mass % or more, and more preferably 2 mass % or more. The upper limit of the Mg content in the hot-dip plated layer is preferably in the range of 8 mass % or less, and more preferably 6 mass % or less.

The hot-dip plated layer may contain Si in the range of 0.0001 to 2 mass %. Si is an element effective for improving the adhesion of the hot-dip plated layer. When Si is incorporated in an amount of 0.0001 mass % or more, an effect of improving adhesion is exhibited, and therefore Si is preferably incorporated in an amount of 0.0001 mass % or more. On the other hand, when the amount of Si in the hot-dip plated layer exceeds 2 mass %, the effect of improving plating adhesion is saturated, so that the amount of Si is set to 2 mass % or less. From the viewpoint of plating adhesion, the lower limit of the Si content in the hot-dip plated layer is more preferably 0.01 mass % or more, still more preferably 0.03% or more. The upper limit of the Si content in the hot-dip plated layer is more preferably 1 mass % or less, and still more preferably 0.8 mass % or less.

In addition, the hot-dip plated layer may contain, as an average composition, any one or two or more of the group consisting of Ni, Ti, Zr, Sr, Fe, Sb, Pb, Sn, Ca, Co, Mn, P, B, Bi, Cr, Sc, Y, REM, Hf, and C in a total amount of 0.0001 to 2 mass %. The total amount is preferably 0.001 mass % or more, or 0.01 mass % or more. In addition, the total amount is up to 2 mass %. By the incorporation of any one or two or more of the group consisting of these elements, corrosion resistance can be further improved. “REM” is one or two or more of the group consisting of rare earth elements having atomic numbers 57 to 71 in the periodic table.

The remainder of the chemical composition of the hot-dip plated layer includes zinc and impurities.

The composition of the hot-dip plated layer can be measured by the following method. First, the surface layer coating film is removed with a coating film remover (for example, NEOREVER SP-751 manufactured by SANSAIKAKO) that does not erode plating, then the hot-dip plated layer is dissolved with hydrochloric acid containing an inhibitor (for example, HIBIRON manufactured by Sugimura Chemical Industry Co., Ltd.), and the obtained solution is subjected to inductively coupled plasma (ICP) optical emission spectroscopy, whereby the composition can be determined.

Next, the microstructure of the hot-dip plated layer will be described. The microstructure of the hot-dip plated layer of the present embodiment may have, for example, the following microstructure.

The hot-dip plated layer containing Al, Mg, and Zn contains an [Al phase], a [MgZnphase], and a [Zn phase], and an [Al/Zn/MgZnternary eutectic structure]. Specifically, it has a form in which an [Al phase], a [MgZnphase], and a [Zn phase] are contained in the matrix of an [Al/Zn/MgZnternary eutectic structure]. When Si is incorporated, a [MgSi phase] may be contained in the matrix of an [Al/Zn/MgZnternary eutectic structure].

The [Al/Zn/MgZnternary eutectic structure] is a ternary eutectic structure of an Al phase, a Zn phase, and an intermetallic compound MgZnphase, and the Al phase forming the ternary eutectic structure corresponds to, for example, an “Al″ phase” (which is an Al solid solution in which Zn is solid-dissolved and contains a small amount of Mg) at a high temperature in an Al—Zn—Mg ternary equilibrium phase diagram.

The Al″ phase at a high temperature usually appears separated into a fine Al phase and a fine Zn phase at normal temperature. The Zn phase in the ternary eutectic structure is a Zn solid solution in which a small amount of Al is solid-dissolved, and in some cases, further a small amount of Mg is solid-dissolved. The MgZnphase in the ternary eutectic structure is an intermetallic compound phase present in the vicinity of Zn: about 84 mass % in a Zn—Mg binary equilibrium phase diagram.

As seen in the phase diagram, it is considered that another additive element is not solid-dissolved in each phase, or even if an additive element is solid-dissolved, the amount of the additive element is extremely small. However, since the amount thereof cannot be clearly distinguished by an ordinary analysis, the ternary eutectic structure formed of these three phases is represented by an [Al/Zn/MgZnternary eutectic structure] in the present specification.

The [Al phase] is a phase that looks like an island with a clear boundary in the matrix of the [Al/Zn/MgZnternary eutectic structure], and this corresponds to, for example, an “Al″ phase” (which is an Al solid solution in which Zn is solid-dissolved and contains a small amount of Mg) at a high temperature in an Al—Zn—Mg ternary equilibrium phase diagram. In the Al″ phase at a high temperature, the amount of Zn and the amount of Mg to be solid-dissolved differ depending on the Al and Mg concentrations in the plating bath. The Al″ phase at a high temperature is usually separated into a fine Al phase and a fine Zn phase at normal temperature, but the island-like shape observed at normal temperature is considered to be attributed to the shape of the Al″ phase at a high temperature.

As seen in the phase diagram, it is considered that another additive element is not solid-dissolved in this phase, or even if an additive element is solid-dissolved, the amount of the additive element is extremely small. However, since it cannot be clearly distinguished by an ordinary analysis, a phase, which is derived from the Al″ phase at a high temperature and whose shape is attributed to the shape of the Al″ phase is referred to as [Al phase] in the present specification.

The [Al phase] can be clearly distinguished from the Al phase forming the [Al/Zn/MgZnternary eutectic structure] by microscopic observation.

The [Zn phase] is a phase that looks like an island with a clear boundary in the matrix of the [Al/Zn/MgZnternary eutectic structure], and in practice, a small amount of Al or a small amount of Mg may be solid-dissolved. As seen in the phase diagram, it is considered that another additive element is not solid-dissolved in this phase, or even if an additive element is solid-dissolved, the amount of the additive element is extremely small.

The [Zn phase] is a region having a circle equivalent diameter of 2.5 μm or more, and can be clearly distinguished from the Zn phase forming the [Al/Zn/MgZnternary eutectic structure] by microscopic observation. The circle equivalent diameter of the [Zn phase] may be 10 μm or less.

The [MgZnphase] is a phase that looks like an island with a clear boundary in the matrix of the [Al/Zn/MgZnternary eutectic structure], and in practice, a small amount of Al may be solid-dissolved. As seen in the phase diagram, it is considered that another additive element is not solid-dissolved in this phase, or even if an additive element is solid-dissolved, the amount of the additive element is extremely small.

The [MgZnphase] and the MgZnphase forming the [Al/Zn/MgZnternary eutectic structure] can be clearly distinguished from each other by microscopic observation. The hot-dip plated layer according to the present embodiment may not contain [MgZnphase] depending on the manufacturing conditions, but it is contained in the hot-dip plated layer under most manufacturing conditions.

The [MgSi phase] is a phase that looks like an island with a clear boundary in a solidified microstructure of the hot-dip plated layer to which Si is added. As seen in the phase diagram, it is considered that Zn, Al, or another additive element is not solid-dissolved in the [MgSi phase], or even if it is solid-dissolved, the amount of thereof is extremely small. The [MgSi phase] can be clearly distinguished from other phases in the hot-dip plated layer by microscopic observation.

Next, the amount of the [Zn phase] will be described. In the present embodiment, as shown in, in a case where a hot-dip plated layeris cut out at any position of a 3t/4 position, a t/2 position, and a t/4 position from the surfaceof the hot-dip plated layerwith the thickness of the hot-dip plated layerformed on a steel sheetrepresented by t, so that exposed surfaces,, andthat are parallel to the surfaceand have a square shape with a 5 mm square size in plane view appear, a ratio (B/A (%)) of an area fraction B of the [Zn phase] to the total area fraction A of the [Zn phase] and the [Al/MgZn/Zn ternary eutectic structure] of the plating microstructure in at least one of the exposed surfacestois 20% or more. The ratio (B/A (%)) may be 25% or more, or 30% or more. When the ratio of the [Zn phase] to the total of the [Zn phase] and the [Al/MgZn/Zn ternary eutectic structure] is 20% or more, the occupancy ratio of a fine white portion exhibiting white increases on the surface of the hot-dip plated layer, so that the entire external appearance of the hot-dip plated layer exhibits white. The upper limit of the ratio (B/A (%)) does not need to be particularly limited, but may be 70% or less, 60% or less, or 55% or less. The schematic cross-sectional view shown inis a cross-sectional view taken along plane A-A′ of.

In at least one of the exposed surfacesto, the area fraction of the [Zn phase] of the plating microstructure is preferably 20% or more. The area fraction of the [Zn phase] may be 25% or more or 30% or more. When the area fraction of the [Zn phase] is 20% or more, the occupancy ratio of a fine white portion exhibiting white increases on the surface of the hot-dip plated layer, so that the entire external appearance of the hot-dip plated layer is closer to white. The upper limit of the area fraction of the [Zn phase] does not need to be particularly limited, but may be 60% or less, 50% or less, or 40% or less.

In at least one of the exposed surfacesto, the average grain size of the [Zn phase] of the plating microstructure is preferably 2.5 to 10 μm. When the average grain size of the [Zn phase] is 2.5 to 10 μm, the surface external appearance can be made closer to white.

In addition, the area fraction of the [Al phase] in the exposed surface for which the area fraction of the [Zn phase] has been measured may be, for example, 10 to 80 area % or 20 to 65 area %.

Further, the area fraction of the [Al/MgZn/Zn ternary eutectic structure] in the exposed surface for which the area fraction of the [Zn phase] has been measured may be, for example, 10 to 80 area % or 20 to 65 area %.

Still further, the area fraction of the [MgZnphase] in the exposed surface for which the area fraction of the [Zn phase] has been measured may be, for example, 0 to 60 area % or 10 to 40 area %.

Still further, the area fraction of the [MgSi phase] in the exposed surface for which the area fraction of the [Zn phase] has been measured may be, for example, 0 to 5 area % or 0 to 1 area %.